Air interface adapting method for a mobile radio system

ABSTRACT

A mobile radio system conveys frames between at least one mobile station and at least one base station, each frame comprising N time slots, a burst of data being transmitted in each time slot, and each time slot being associated with a separate call between a mobile station and a base station. For each call over a transmission channel, the quality of the transmission channel is analyzed and, on the, basis thereof, a selection is made between first and second modes of transmission. In the first mode of transmission data to be transmitted is organized in first blocks obtained by a first encoding of source blocks, each of the first blocks has a size greater than the content of (P-1) bursts, where P is an even number at least equal to 2, and the data of the same first block is distributed between at least P bursts. In the second mode of transmission the data to be transmitted is organized in second blocks each having a size less than or equal to the content of P/2 bursts, the data of the same second block is distributed into at most P/2 bursts, at least P/2 bursts are available relative to the first mode of transmission, and an automatic retransmission request mechanism is used to retransmit in the available bursts second blocks that are detected to have been received incorrectly. This second mode of transmission necessitates a quality of the transmission channel greater than that necessitated by the first mode of transmission.

BACKGROUND OF THE INVENTION Field of the Invention

The field of the invention is that of digital mobile radio systems. Theinvention applies in particular to cellular mobile radio systems such asGSM (Global System for Mobile communications) systems.

More particularly, the invention concerns the communication of digitaldata in systems utilizing time-division multiplexing, employing the TDMA(Time Division Multiple Access) technique.

The TDMA technique divides time into frames of predetermined fixedduration, the frames being in turn divided into time slots. One or moretime slots are associated with each call. A burst can be transmitted ineach time slot.

A frame comprises N time slots (i.e. N bursts) that can correspond to Ncalls. Each receiver is able to extract the bursts corresponding to thetime slots that are addressed to it, in order to reconstruct the sourcesignal. In this way, N calls can be transmitted in the same frequencyband.

After transmission on a radio channel, in particular a mobile radiochannel using frequency hopping and with a quasirandom distribution ofthe interference signals, the quality of the frames (and therefore ofthe bursts that they contain) is variable. The transmission channel maybe subject to various disturbances, including rapid fading and impulsiveinterference, which cause sequences of errors in the frames.

To attempt to solve this problem, in one standard mode of transmissionthe data to be transmitted is organized into source blocks and isencoded and then distributed (i.e. interleaved) into bursts each ofwhich belongs to a separate frame.

The encoding process, usually called error control coding or channelcoding, is intended to protect the wanted data to be transmitted byjudiciously converting it into encoded data. An encoded block isgenerally larger than a source block. At the receiver, the encoded datais processed to recover the wanted data, even in the presence oftransmission errors due to the poor quality of the transmission channel.

The interleaving that follows encoding consists in dividing the data ofthe same encoded block between several bursts belonging to separateframes, so that they are transmitted at separate times. At the receiver,decoding is preceded by deinterleaving which breaks up the series oferrors and, ideally, produces isolated errors that are more easilycorrected upon decoding the codes used.

In the standard mode of transmission this interleaving, i.e. thedistribution of the data from the same encoded block between severalbursts, is mandatory since the size of the encoded blocks means that itis not possible to place all of an encoded block in one burst, inparticular because they contain a relatively large amount of redundantdata. The encoding process, and thus the amount of redundant data, arechosen to enable correct decoding when transmission channel quality ispoor (i.e. in the worst case scenario).

This standard mode of transmission has various disadvantages, eventhough it enables effective and correct exchange of data regardless ofthe quality of the transmission channel.

First of all, it is clear that the standard mode of transmission, beingdesigned to work in the worst case scenario, is not optimized when thetransmission channel quality is reasonably good.

The interleaving at the transmitter implies a time-delay at the receiverwhich can be relatively large, since the corresponding deinterleavingprocess presupposes the arrival of several successive frames before anencoded block can be reconstituted.

An object of the invention is to overcome these various disadvantages ofthe prior art.

To be more precise, one object of the present invention is to provide amethod of adapting the air interface in a mobile radio system tooptimize the exchange of data bursts contained in frames, and inparticular to reduce the amount of data transmitted.

Another object of the invention is to provide a method of this kind thatreduces the time-delays.

A further object of the invention is to provide a method of this kindenabling the use of transmission resources (i.e. channels) to beoptimized and thus to limit the interference induced in nearby cellsusing the same frequency band. This object of reducing interference isparticularly important in cellular mobile radio systems. In thesesystems, the same frequency band is allocated to several geographicallydispersed cells. Although the distribution of the latter is defined in away that maximizes the distance between the cells, it is not rare forthe signals of a given cell to experience interference from those ofother cells using the same band.

SUMMARY OF THE INVENTION

These objects, and others that emerge hereinafter, are achieved inaccordance with the invention by means of a method of adapting airinterface, in a mobile radio system conveying, between at least onemobile station and at least one base station frames each made up of Ntime slots, a burst of data being transmitted in each time slot, eachtime slot being associated with a separate call between a mobile stationand a base station, said system providing a first mode of transmissionin which the data of a call to be transmitted is organized in firstblocks obtained by first encoding of source blocks, each of said firstblocks having a size greater than the content of (P-1) burst(s) where Pis an even number at least equal to 2, the data of the same first blockbeing divided between at least P bursts,

said system providing a second mode of transmission in which the data ofa call to be transmitted is organized in second blocks each having asize less than or equal to the content of P/2 burst(s), the data of thesame second block being divided between at most P/2 burst(s), at leastP/2 burst(s) being available relative to the first mode of transmission,and an automatic retransmission request mechanism being implemented toretransmit in said available bursts second blocks detected to have beenreceived incorrectly,

and, for each call between a mobile station and a base station via atransmission channel, the quality of said transmission channel isanalyzed and one of said modes of transmission is selected according tosaid analysis of said quality of the transmission channel, said secondmode of transmission necessitating a level of quality of thetransmission channel greater than that necessitated by said first modeof transmission.

The principle of the invention is therefore to use selectively either afirst or a second mode of transmission. The first mode of transmissioncorresponds to the standard mode of transmission previously discussed.The second mode uses the burst by burst ARQ (Automatic Repeat Request)transmission technique. Thus the invention takes advantage of the factthat, with a channel of sufficient quality, the "burst by burst ARQ"technique (second mode of transmission) is more efficient than theencoding-interleaving technique (first mode of transmission).

In other words, if the quality of the channel is sufficient, the secondmode of transmission is selected and the exchange of data is optimized:

the amount of data transmitted is reduced since there is no encoding(and therefore no redundancy) or only "light" encoding (leading to lessredundancy than the encoding process used is the first mode oftransmission); note that minimal encoding is generally retained in orderto enable error detection; and

the time-delays are reduced since there is no (or less) interleaving(instead of distribution into at least P bursts (i.e. at least P frames)there is distribution into at most P/2 bursts (i.e. P/2 frames));sufficient channel quality is obtained, for example, in an indoorenvironment.

When the quality of the channel is somewhat poor, on the other hand, thefirst mode of transmission is more suitable since, by using "robust"encoding and deeper interleaving, it can correct a relatively largenumber of errors.

The "burst by burst ARQ" technique is usually implemented in thefollowing manner: the time interval Δt separating the sending of twosuccessive bursts is of constant duration, except when a burst isretransmitted. In this case, if the nth burst (for example) is detectedto have been received incorrectly, it is retransmitted at the timeintended for transmitting the (n+1)th burst (i.e. Δt after transmissionof the nth burst). An additional transmission resource is thereforefreed during a subsequent time slot so that the (n+1)th burst istransmitted before the time intended for the (n+2)th burst, which istransmitted on schedule 2Δt after transmission of the nth burst.

In the advantageous embodiment of the invention, the "burst by burstARQ" technique is implemented differently. In the second mode oftransmission each frame comprises one burst for each call and there areat least P/2 bursts available. These available bursts can therefore beused to retransmit bursts detected to have been received incorrectly. Inother words, the system does not have to free any additionaltransmission resource and all the bursts (including those that areretransmitted) can be transmitted at regular time intervals (i.e. everywanted frame, unless retransmission is required). If the nth packet isdetected to have been received incorrectly, for example, it isretransmitted during the time slot intended for transmission of the(n+1)th burst, this time slot being available since the change from thefirst mode of transmission to the second frees bursts (i.e. time slots).The (n+2)th burst can therefore be transmitted normally 2Δt after thetransmission of the nth burst.

Moreover, in this advantageous embodiment of the invention interferenceis limited since one burst in two is not used (unless a second block hasto be retransmitted) and therefore cannot generate interference.

Said mobile radio system is advantageously of the GSM (Global System forMobile communications) type and said second mode of transmission isadvantageously of the half-rate type, a second block having a size lessthan or equal to the content of a burst, only one frame in two beingused for a given call.

In this case, a second block is sufficiently small to fit in a burst.Over a series of four consecutive frames, a given time slot in the firstand third frames is assigned to a first call and the same time slot inthe second and fourth frames is assigned to a second call. For the firstcall, for example, the advantageous embodiment of the invention placesin the corresponding time slot of the first frame a burst containing asecond block, the corresponding time slot of the third frame beingeither empty or used to retransmit in a burst the second blockpreviously transmitted in the first frame if the second block isdetected to have been received incorrectly.

The data from the same first block is advantageously divided between atleast four bursts. The data from the same second block is in the sameburst.

Said first mode of transmission is preferably selected from:

a full-rate type first mode of transmission in which a first block has asize substantially equal to the content of four bursts, the data of thesame first block being distributed between eight bursts, at the rate ofapproximately one eighth of the first block per burst;

a half-rate type first mode of transmission in which a first block has asize substantially equal to the content of two bursts, the data of thesame first block being divided into four bursts, at the rate ofapproximately one quarter of said first block per burst, only one framein two being used for the same call.

In other words, the first mode of transmission is preferably either afull-rate GSM mode or a half-rate GSM mode.

In a first advantageous embodiment of the invention, said first mode oftransmission being of the half-rate type,

a first quarter of a given first block being placed in a burst of an ithframe with a third quarter of a preceding first block,

a second quarter of said given first block being placed in a burst of an(i+2)th frame with a fourth quarter of said preceding first block, athird quarter of said given first block being placed in a burst of an(i+4)th frame with a first quarter of a preceding first block,

a fourth quarter of said given first block being placed in a burst of an(i+6)th frame with a second quarter of said preceding first block,

when said second mode of transmission is selected, the change from saidfirst mode of transmission to the second consists in, for each call,taking as the last burst transmitted in the first mode of transmission aburst in which are placed a fourth quarter of a given first block and asecond quarter of the next first block, said next first block beingconsidered by the receiver not to have been transmitted since the thirdand fourth quarters of said next first block are never transmitted,

and in that, when said first mode of transmission is selected, thechange from said second mode of transmission to the first consists in,for each call, taking as the first burst transmitted in the first modeof transmission a burst in which is placed a first quarter of a givenfirst block.

In a second advantageous embodiment of the invention said first mode oftransmission is again of the half-rate type and when said second mode oftransmission is selected, the change from said first mode oftransmission to the second consists in, for each call:

dividing a given second block in two;

placing in a burst corresponding to this call and which is thepenultimate burst transmitted in the first mode of transmission a thirdquarter of a given first block and a first half of said given secondblock;

placing in a burst corresponding to this call and which is the lastburst transmitted in the first mode of transmission a fourth quarter ofsaid given first block and a second half of said given second block.

Preferably, when said first mode of transmission is selected, the changefrom said second mode of transmission to the first consists in, for eachcall:

constructing an artificial first block;

dividing a given second block in two;

placing in a burst corresponding to this call and which is thepenultimate burst transmitted in the second mode of transmission a firsthalf of said given second block and a first quarter of said artificialfirst block;

placing in a burst corresponding to this call and which is the lastburst transmitted in the second mode of transmission a second half ofsaid given second block and a second quarter of said given first block;

placing in a burst corresponding to this call and which is the firstburst transmitted in the first mode of transmission a third quarter ofsaid artificial first block and a first quarter of a given first block;

placing in a burst corresponding to this call and which is the secondburst transmitted in the first mode of transmission a fourth quarter ofsaid artificial first block and a second quarter of said given firstblock.

Accordingly, in the first and second embodiments the first mode oftransmission is a half-rate GSM mode and the second mode of transmissionis a half-rate "burst by burst ARQ" mode.

In the first advantageous embodiment of the invention, the change fromone mode of transmission to the other is effected "brutally" and resultsin the loss of a first block (during a "change" of first block). In thesecond advantageous embodiment of the invention, on the other hand, thechange from one mode of transmission to the other is effected "gently"since there is no loss of any first block (thanks to interleaving offirst and second blocks on changing from one mode of transmission to theother).

Said data to be transmitted is advantageously speech data.

Preferably, the change from the first mode of transmission to the secondinducing a transmission time shift enabling faster processing at thereceiver, at the transmitter, a speech signal corresponding to saidspeech data is analyzed, and said transmission time shift is eliminatedin a deferred manner according to said analysis of the speech signal,with the aim of minimizing disturbances of a speech signal reconstitutedin the receiver due to said change from the first mode of transmissionto the second.

Accordingly, on changing from the first mode of transmission to thesecond, the decision is taken to reduce the transmission time shift(this is also referred to as reducing the time delay). There is atransmission time shift because, in the first mode of transmission, afirst block can be reconstituted and processed only after a time periodcorresponding to the reception of six frames and one time slot, whereasin the second mode of transmission, in the worst case scenario (i.e. ifthe second block must be retransmitted), a second block can bereconstituted after a time corresponding to the reception of two framesand one time slot (one time slot only, if the second block does not haveto be retransmitted).

To reduce this time-delay without excessive disturbance to the qualityof the call (i.e. the quality of the speech signal), the reduction ofthe time-delay is deferred to a period in which there is littlevariation of the speech signal, i.e. a period in which the eliminationof a speech block is not very perceptible to those speaking.

Said analysis of a speech signal advantageously consists in determiningspectral coefficients of said speech signal.

Said second blocks are advantageously obtained by the second encoding ofsaid source blocks. Accordingly, error correction is obtained not onlyby the retransmission of the second block detected to have been receivedincorrectly but also by encoding/decoding. In other words, by encodingthe wanted data, even in the second mode of transmission using the"burst by burst ARQ" technique, an additional level of correction isadded and the reliability of the received data is improved. It isnevertheless clear that the encoding referred to here is less powerfulthan that used in the standard mode of transmission, since a (second)block must be able to fit entirely within a burst, rather than beingdivided between a plurality of successive bursts (like the firstblocks).

Preferably, during operation in said first mode of transmission, theanalysis of the quality of the transmission channel consists indetermining at least one first information selected from:

the number of bursts detected to have been received incorrectly;

an estimate of the bit error rate;

an estimate of the impulse response of the transmission channel;

an estimate of the signal to noise ratio; and, during operation in saidsecond mode of transmission, the analysis of the quality of thetransmission channel consists in determining at least one secondinformation selected from:

the number of bursts retransmitted;

the number of bursts retransmitted detected to have been receivedincorrectly;

an estimate of the bit error rate;

an estimate of the impulse response of the transmission channel;

an estimate of the signal to noise ratio.

Preferably, during operation in said first mode of transmission orrespectively said second mode of transmission, the selection of thesecond or respectively the first mode of transmission comprises a stepof comparing the first or respectively second information representativeof the quality of the transmission channel with at least one first orrespectively second predetermined threshold.

Advantageously, during operation in said first mode of transmission, thesecond mode of transmission is selected if said first informationrepresentative of the quality of the transmission channel is below saidpredetermined first threshold, and, during operation in the second modeof transmission, the first mode of transmission is selected if saidsecond information representative of the quality of the transmissionchannel is above said predetermined second threshold.

Advantageously, an indication of the change from the first mode oftransmission to the second is transmitted over an auxiliary channelreserved for protocol data. In the case of a GSM system, one suchauxiliary channel is an FACCH channel, for example.

An indication of the change from the second mode of transmission to thefirst is preferably placed in one of said available bursts.

Other features and advantages of the invention will emerge from areading of the following description of one preferred embodiment of theinvention given by way of illustrative and non-limiting example and fromthe accompanying drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic representation of one example of a mobile radiosystem in which the method of the invention can be used;

FIG. 2 is a flowchart of the principle of the method of the invention;

FIGS. 3 and 4 each show a respective different mode of transmission thatcan be selected when the method of the invention is used;

FIG. 5 is a simplified diagrammatic representation of half-rateoperation of the mode of transmission shown in FIG. 4;

FIG. 6 is a flowchart of the method of the invention; and

FIGS. 7, 8 and 9 each show one example of the distribution of data to betransmitted in bursts, respectively in the case of a full-rate GSM modeof transmission, a half-rate GSM mode of transmission and a half-rate"burst by burst ARQ" mode of transmission.

The invention concerns a method of adapting the air interface (i.e. thephysical and link layers (levels 1 and 2 of the ISO's OSI model)) in amobile radio system enabling exchange of frames of data, and inparticular of frames of speech data, using the TDMA (Time DivisionMultiple Access) technique. Each frame comprises N time slots, a burstof data being transmitted in each time slot, and each time slot beingassociated with a separate call between a mobile station and a basestation.

FIG. 1 is a diagrammatic representation of a type of cellular networkthat is known in itself. The geographical territory covered by themobile radio system is divided into cells 11₁ through 11_(n). Each cell11₁ contains a base station 12₁ capable of exchanging signal frames 13₁,13₂ with a plurality of mobile stations 14₁, 14₂ moving around in saidcell 11₁.

Two adjoining cells 11₁, 11₂ use separate frequency bands so that thereis no interference between the signals transmitted in the two cells. Tobe more precise, the allocation of frequencies is based on anorganization of the cells into patterns 15₁, 15₂ each of seven cells.Within the same pattern, the frequencies allocated are different. Theyare re-used from one pattern 15₁ to the other 15₂, on the other hand.For example, the cell 11₅ uses the same frequency as the cell 11₁.

As shown by the flowchart in FIG. 2, the principle of the method of theinvention is as follows: for each call between a mobile station 14₁, 14₂and a base station 12₁, 12₅ via a transmission channel the quality ofthe transmission channel is analyzed (step 21) and one of at least twopossible modes of transmission is selected (step 22) according to theresult of the preceding analysis (step 21). In other words, the burstsof the same frame can be associated with different (first or second)modes of transmission since each burst of a frame corresponds to aseparate call.

The two modes can differ in the manner in which, for a given call, theblocks of data to be transmitted are distributed in the bursts containedin the frames and in the manner of correcting transmission errors.

A first mode of transmission that can be selected in a standard modeshown in FIG. 3. In this standard mode the data 31 to be transmitted isorganized into encoded blocks which are the result of encoding (step 32)wanted data 33 which is itself organized into source blocks. Eachencoded block, which is therefore made up of encoded data (generallymore than the wanted data), has a size greater than the capacity of(P-1) bursts 35, where P is an even number at least equal to 2.Consequently, the data 31 in the same encoded block is divided (i.e.interleaved) (step 34) between at least P bursts 35. In many instancesP=2.

Transmission error correction consists in, at the receiver,deinterleaving (i.e. reconstituting the encoded blocks) followed bydecoding (the encoded blocks reconstituted in this way). This first modeof transmission is more reliable if the encoding is more powerful (sothat the amount of encoded data is large) and the depth of interleaving(i.e. the number of bursts between which the data from the encoded blockis divided) is greater. Note that the depth of interleaving alsocorresponds to a number of frames between which an encoded block isdistributed since, for a given call, each frame comprises a burst in thetime slot reserved to that call.

This standard mode is therefore capable of working even if thetransmission channel quality is poor (i.e. even if the data beforedeinterleaving and decoding contains a relatively large number oferrors). On the other hand, this standard mode requires the sending of arelatively large amount of data because of the encoding, which is to thedetriment of the resources available for the wanted data, and introducestime-delays, due to the interleaving.

A second mode of transmission that can be selected is a "burst by burstARQ" mode of transmission. In this second mode of transmission, shown inFIG. 4, the data 36 to be transmitted is organized in blocks each havinga size less than (or equal to) the content of P/2 burst(s) 37.Consequently, the data 36 from the same block is placed (step38)--without interleaving it--in at most P/2 bursts 37.

There are therefore at least P/2 burst(s) available compared to thefirst mode of transmission, and these bursts are used to retransmitbursts detected to have been received incorrectly.

Note that in the example shown in FIG. 4 the blocks of data 36 to betransmitted are obtained by encoding (step 39) wanted data 310. However,the invention also concerns the situation in which the wanted data 310directly constitutes the data 36 to be transmitted. Generally, the data36 to be transmitted can be produced by any form of processing, providedthat the size of a block of data 36 to be transmitted is always lessthan the content of P/2 burst(s) 37.

In the second mode of transmission, transmission errors are corrected byretransmitting the bursts (and therefore the blocks, since a burstcontains a block) detected to have been received correctly. Using thissecond mode of transmission economizes on transmission resources whenthe quality of the channel is good. In this case, the number of burststo be retransmitted is small. This second mode of transmission alsoreduces the time-delays since there is no interleaving.

By dynamically selecting the more appropriate mode of transmission,depending on the quality of the transmission channel (i.e. the firstmode of transmission if the quality is somewhat poor and the second ifit is somewhat good), the method of the invention optimizes the use ofresources and reduces the time-delays where possible.

The remainder of the description considers the case of a GSM (GlobalSystem for Mobile communications) type mobile radio system). It isnevertheless clear that the invention is not limited to this particularsystem but concerns more generally all mobile radio systems.

FIGS. 7 and 8 each show one example of distribution of data to betransmitted into bursts, respectively in the case of a full-rate GSM anda half-rate GSM first mode of transmission.

In the case of the full-rate GSM type (see FIG. 7), a first block (71₁-71₃) has a size substantially equal to the content of four bursts (i.e.four time slots; only the first time slot IT1 of each frame 72₁ through72₈ is shown). The data of the same first block is divided between eightbursts, at the rate of approximately one eighth of the first block perburst. Thus in FIG. 7 the distribution is as follows:

a first eighth of the first block n°2 is placed in a burst of frame n°1with a fifth eighth of the first block n°1,

a second eighth of the first block n°2 is placed in a burst of frame n°2with a sixth eighth of the first block n°1,

a third eighth of the first block n°2 is placed in a burst of frame n°3with a seventh eighth of the first block n°1,

a fourth eighth of the first block n°2 is placed in a burst of frame n°4with an eighth eighth of the first block n°1,

a fifth eighth of the first block n°2 is placed in a burst of frame n°5with a first eighth of the first block n°3,

a sixth eighth of the first block n°2 is placed in a burst of frame n°6with a second eighth of the first block n°3,

a seventh eighth of the first block n°2 is placed in a burst of framen°7 with a third eighth of the first block n°3,

an eighth eighth of the first block n°2 is placed in a burst of framen°8 with a fourth eighth of the first block n°3.

In the case of the half-rate GSM type (see FIG. 8), a first block (81₁-81₃) has a size substantially equal to the content of two bursts (i.e.two time slots; only the first time slot IT1 of each frame 82₁ through82₈ is shown). The data from the same first block is divided betweenfour bursts, at the rate of approximately one quarter of the first blockper burst, and only one frame in two 82₁, 82₃, 82₅, 82₇ is used for thesame call. Thus in the FIG. 8 example the distribution is as follows:

a first quarter of the first block n°2 is placed in a burst of frame n°1with a third quarter of the first block n°1,

a second quarter of the first block n°2 is placed in a burst of framen°3 with a fourth quarter of the first block n°1,

a third quarter of the first block n°2 is placed in a burst of frame n°5with a first quarter of the first block n°3,

a fourth quarter of the first block n°2 is placed in a burst of framen°7 with a second quarter of the first block n°3,

FIG. 5 is a simplified diagrammatic representation of the operation ofone particular embodiment of the "burst by burst ARQ" mode oftransmission (the second mode of transmission shown in FIG. 4), namely ahalf-rate embodiment. FIG. 9 shows in detail the distribution of thedata to be transmitted in the bursts in this case of a half-rate "burstby burst ARQ" mode of transmission.

In the half-rate "burst by burst ARQ" mode of transmission (see FIG. 9)a second block (91₁ through 91₃) has a size substantially equal to thecontent of a burst (i.e. a time slot; only the first time slot IT1 foreach frame 92₁ through 92₈ is shown). The data from the same secondblock is all placed in the same burst and only one frame in two 92₁,92₃, 92₅, 92₇, 92₉, 92₁₁ is used for the same call. In the FIG. 9example the distribution is therefore as follows:

block n°1 is placed in a burst of frame n°1,

block n°2 is placed in a burst of frame n°5,

block n°3 is placed in a burst of frame n°9.

Bursts received incorrectly are retransmitted as follows:

if the burst of frame n°1 is received incorrectly, block n°1 is sentagain in a burst of frame n°3,

if the burst of frame n°5 is received incorrectly, block n°2 is sentagain in a burst of frame n°7,

if the burst of frame n°9 is received incorrectly, block n°3 is sentagain in a burst of frame n°11.

FIG. 5 shows the two transmission channels between a base station and amobile station, namely a "mobile station to base station" channel("uplink") and a "base station to mobile station" channel ("downlink").For simplicity, empty packets are not shown except for those used toretransmit a block detected to have been received incorrectly. Thefollowing discussion refers only to the bursts corresponding to the samecall.

In this example, the first two bursts 37₁, 37₃ containing a block ofdata 51₁, 51₂ are transmitted correctly on the downlink. However, thethird burst 375 containing a block of data 51₃ is not transmittedcorrectly. The receiver advises the sender of this and the latterresends the block 51₃ in the burst 37₆.

In the example given for the uplink, the first burst 37'₁ is detected tohave been received incorrectly and its content is then retransmitted inthe next burst 37'₂ without disturbing the transmission of the otherbursts 37'₃, 37'₅, 37'₅ containing blocks of data to be transmitted.

This embodiment of the half-rate second mode of transmission limitsinterference 16 due to the use of the same frequencies in differentcells 11₁, 11₅. For one burst in two 92₃, 92₇, 92₁₁ no signals are sent(except for a block that is retransmitted) and therefore there is nointerference 16.

FIG. 6 is a flowchart of the method of the invention.

At a given time, for a given call, the system is assumed to be operatingin the first mode of transmission (step 61). At regular time intervals,for example after transmission of a predetermined number of bursts, thequality of the transmission channel is analyzed (step 62) for the callin question. Following this analysis (step 62), the estimated quality iscompared (step 63) to a predetermined quality threshold. If theestimated quality is below that predetermined quality threshold for thecall in question the system continues to operate in the first mode oftransmission (step 61). Otherwise the second mode of transmission isselected for this call (step 64) and operation in the second mode oftransmission begins (step 65). When the system is operating in thesecond mode of transmission (step 65), the quality of the transmissionchannel is analyzed (step 66) at regular time intervals. After thisanalysis (step 66), the estimated quality is compared (step 67) with theaforementioned quality threshold. If the estimated quality is above orequal to the predetermined quality threshold, for this call the systemcontinues to operate in the second mode of transmission (step 65).Otherwise, the first mode of transmission is selected (step 68) and, forthe call in question, operation is thereafter in the first mode oftransmission (step 61).

Various ways of achieving this change from one mode of transmission tothe other are described below.

When the system is operating in the first mode of transmission (step61), the analysis (step 62) of the quality of the transmission channelconsists in determining the number of bursts detected to have beenreceived incorrectly, for example. Comparing (step 63) the estimatedquality of the channel to a predetermined quality threshold then amountsto comparing the number of packets detected to have been receivedincorrectly (as a percentage, i.e. for a given number of burstsreceived) to a predetermined threshold number. Note that the directionof the inequality is reversed (relative to that concerning the qualityof the channel) since the number of bursts detected to have beenreceived incorrectly is inversely proportional to the quality of thetransmission channel. The change from the first mode of transmission tothe second can be made, for example, when the percentage of burstsreceived incorrectly is below 1% (calculated for the last 500 bursts,for example, which takes 10 s in the case of the GSM system).

It is clear that in the case of the first mode of transmission there aremany other ways to analyze the quality of the transmission channel. Thefollowing parameters may be used, for example: an estimate of the biterror rate, an estimate of the impulse response of the channel or anestimate of the signal to noise ratio.

When the system is operating in the second mode of transmission (step65), the analysis (step 66) of the quality of the transmission channelconsists in determining the number of bursts retransmitted, for example.Once again, the comparison (step 67) of the estimated quality of thechannel to a predetermined quality threshold amounts to comparing thenumber of bursts retransmitted (as a percentage, for a given number ofbursts transmitted) to a predetermined threshold number. The first modeof transmission is selected only if the number of bursts retransmittedis greater than the predetermined threshold number, i.e. if theestimated quality is below a predetermined threshold value (once again,the inequalities are reversed). The change from the second mode oftransmission to the first can take place, for example, when at least oneburst in six has to be retransmitted or if the number of burstsretransmitted detected to have been received incorrectly is greater than3% (calculated over the last 60 bursts, for example, which takes 1.2 sin the case of the GSM system).

In the case of the second mode of transmission, the quality oftransmission can also be analyzed by measuring the following parameters:the number of bursts retransmitted detected to have been receivedincorrectly, an estimate of the bit error rate, an estimate of theimpulse response of the transmission channel or an estimate of thesignal to noise ratio.

The indication of the change from the first mode of transmission to thesecond is transmitted over an auxiliary channel reserved for protocoldata, for example. This can be the FACCH channel in the case of the GSMsystem, for example. The indication of the change from the second modeof transmission to the first is placed in an available burst 37₂, 37₄,37₆, 37₈ for example.

Consider now the change from one mode of transmission to the other, inthe particular case in which the first mode of transmission is ahalf-rate GSM mode (see FIG. 8) and the second mode of transmission is ahalf-rate "burst by burst ARQ" mode (see FIG. 9).

A first solution proposed by the invention is as follows:

When the second mode of transmission is selected, changing from thefirst mode of transmission to the second, taking as the last bursttransmitted in the first mode of transmission a burst in which areplaced a fourth quarter of a given first block and a second quarter ofthe next first block. Consequently, the next first block is consideredby the receiver not to have been transmitted since the third and fourthquarters of this next first block are never transmitted. In other words,a block is lost.

When said first mode of transmission is selected, changing from thesecond mode of transmission to the first, taking as the first bursttransmitted in the first mode of transmission a burst in which is placeda first quarter of a given first block. The receiver then creates afirst block artificially in order to fill in the transmission time-delaydue to the change of mode of transmission.

This first solution is somewhat "brutal" since it leads to a loss ofblock in one direction and the addition of a block in the otherdirection.

A second solution proposed by the invention is more flexible. In thissecond solution, when the second mode of transmission is selected, thechange from the first mode of transmission to the second consists in,for each call:

dividing a given second block in two;

placing in a burst corresponding to this call and which is thepenultimate burst transmitted in the first mode of transmission a thirdquarter of a given first block and a first half of said given secondblock;

placing in a burst corresponding to this call and which is the lastburst transmitted in the first mode of transmission a fourth quarter ofsaid given first block and a second half of said given second block.

There is therefore a "flexible" change from the first mode oftransmission to the second since the last "first block" is interleavedwith the first "second block". In other words, no block is lost onchanging from the first mode of transmission to the second.

In this second solution, when the first mode of transmission isselected, the change from the second mode of transmission to the firstconsists in, for each call:

constructing an artificial first block;

dividing a given second block in two;

placing in a burst corresponding to this call and which is thepenultimate burst transmitted in the second mode of transmission a firsthalf of said given second block and a first quarter of said artificialfirst block;

placing in a burst corresponding to this call and which is the lastburst transmitted in the second mode of transmission a second half ofsaid given second block and a second quarter of said given first block;

placing in a burst corresponding to this call and which is the firstburst transmitted in the first mode of transmission a third quarter ofsaid artificial first block and a first quarter of a given first block;

placing in a burst corresponding to this call and which is the secondburst transmitted in the first mode of transmission a fourth quarter ofsaid artificial first block and a second quarter of said given firstblock.

There is therefore a "flexible" change from the second mode oftransmission to the first since the last "second block" is interleavedwith a first "first block" constructed artificially by the transmitter(for example by interpolation of speech).

The invention also proposes to eliminate the transmission time shift(i.e. to reduce the time-delay) induced by the change from the firstmode of transmission to the second.

At the transmitter, for example, a speech signal corresponding to saidspeech data is analyzed and the elimination of the time-delay isdeferred on the basis of this analysis of the speech signal, so as tominimize disturbances of a speech signal reconstituted at the receiverdue to the change from the first mode of transmission to the second. Theanalysis of a speech signal consists in determining spectralcoefficients of the speech signal, for example, and deducing therefromthe periods of relative stability of the speech signal. The elimination(or reduction) of the time-delay, which consists in eliminating a block,is then carried out as soon as any such period is detected.

We claim:
 1. A method of adapting an air interface, in a mobile radiosystem conveying, between at least one mobile station and at least onebase station frames each made up of N time slots, a burst of data beingtransmitted in each time slot, and each time slot being associated witha separate call between a mobile station and a base station, said systemproviding a first mode of transmission in which the data of a call to betransmitted is organized in first blocks obtained by first encoding ofsource blocks, each of said first blocks having a size greater than thecontent of (P-1) burst(s) where P is an even number at least equal to 2,the data of a same first block being divided between at least Pbursts,wherein said system provides a second mode of transmission inwhich said data of a call to be transmitted is organized in secondblocks each having a size less than or equal to the content of P/2burst(s), the data of a same second block being divided between at mostP/2 burst(s), at least P/2 burst(s) being available relative to saidfirst mode of transmission, and an automatic retransmission requestmechanism being implemented to retransmit in said available burstssecond blocks detected to have been received incorrectly, and, for eachcall between a mobile station and a base station via a transmissionchannel, the quality of said transmission channel is analyzed and one ofsaid modes of transmission is selected according to said analysis ofsaid quality of said transmission channel, said second mode oftransmission necessitating a level of quality of said transmissionchannel greater than that necessitated by said first mode oftransmission.
 2. The method claimed in claim 1 wherein said mobile radiosystem is of a GSM (Global System for Mobile communications) type, andsaid second mode of transmission is of a half-rate type, said secondblock having a size less than or equal to the content of a burst, onlyone frame in two being used for a given call.
 3. The method claimed inclaim 2 wherein the data of the same first block is distributed betweenat least four bursts, and the data of the same second block is in thesame burst.
 4. The method claimed in claim 3 wherein said first mode oftransmission is selected from:a full-rate type first mode oftransmission in which a first block has a size substantially equal tothe content of four bursts, the data of the same first block beingdistributed between eight bursts, at the rate of approximately oneeighth of the first block per burst; and a half-rate type first mode oftransmission in which a first block has a size substantially equal tothe content of two bursts, the data of the same first block beingdivided into four bursts, at the rate of approximately one quarter ofsaid first block per burst, only one frame in two being used for thesame call.
 5. The method claimed in claim 4, wherein said first mode oftransmission is of the half-rate type, a first quarter of a given firstblock is placed in a burst of an ith frame with a third quarter of apreceding first block, a second quarter of said given first block isplaced in a burst of an (i+2)th frame with a fourth quarter of apreceding first block, a third quarter of said given first block isplaced in a burst of an (i+4)th frame with a first quarter of apreceding first block, and a fourth quarter of said given first block isplaced in a burst of an (i+6)th frame with a second quarter of saidpreceding first block, when said second mode of transmission isselected, the change from said first mode of transmission to the secondconsists in, for each call, taking as the last burst transmitted in saidfirst mode of transmission a burst in which are placed a fourth quarterof a given first block and a second quarter of the next first block,said next first block being considered by the receiver not to have beentransmitted since the third and fourth quarters of said next first blockare never transmitted, and, when said first mode of transmission isselected, the change from said second mode of transmission to the firstconsists in, for each call, taking as the first burst transmitted insaid first mode of transmission a burst in which is placed a firstquarter of a given first block.
 6. The method claimed in claim 4,wherein said first mode of transmission is of the half-rate type, afirst quarter of a given first block is placed in a burst of an ithframe with a third quarter of a preceding first block, a second quarterof said given first block is placed in a burst of an (i+2)th frame witha fourth quarter of said preceding first block, a third quarter of saidgiven first block is placed in a burst of an (i+4)th frame with a firstquarter of a next first block, and a fourth quarter of said given firstblock is placed in a burst of an (i+6)th frame with a second quarter ofsaid next block, when said second mode of transmission is selected, thechange from said first mode of transmission to the second consists in,for each call:dividing a given second block in two; placing in a burstcorresponding to this call and which is the penultimate bursttransmitted in the first mode of transmission a third quarter of a givenfirst block and a first half of said given second block; and placing ina burst corresponding to this call and which is the last bursttransmitted in the first mode of transmission a fourth quarter of saidgiven first block and a second half of said given second block.
 7. Themethod claimed in claim 6 wherein, when said first mode of transmissionis selected, the change from said second mode of transmission to thefirst consists in, for each call:constructing an artificial first block;dividing a given second block in two; placing in a burst correspondingto this call and which is the penultimate burst transmitted in saidsecond mode of transmission a first half of said given second block anda first quarter of said artificial first block; placing in a burstcorresponding to this call and which is the last burst transmitted insaid second mode of transmission a second half of said given secondblock and a second quarter of said given first block; placing in a burstcorresponding to this call and which is the first burst transmitted insaid first mode of transmission a third quarter of said artificial firstblock and a first quarter of a given first block; and placing in a burstcorresponding to this call and which is the second burst transmitted insaid first mode of transmission a fourth quarter of said artificialfirst block and a second quarter of said given first block.
 8. Themethod claimed in claim 1 wherein said data to be transmitted is speechdata.
 9. The method claimed in claim 8 wherein the change from saidfirst mode of transmission to the second induces a transmission timeshift enabling faster processing at the receiver, at the transmitter, aspeech signal corresponding to said speech data is analyzed, and saidtransmission time shift is eliminated in a deferred manner according tosaid analysis of said speech signal, with the aim of minimizingdisturbances of a speech signal reconstituted in the receiver due tosaid change from said first mode of transmission to the second.
 10. Themethod claimed in claim 9 wherein said analysis of a speech signalconsists in determining spectral coefficients of said speech signal. 11.The method claimed in claim 1 wherein said second blocks are obtained bya second encoding of said source blocks.
 12. The method claimed in claim1 wherein:during operation in said first mode of transmission, theanalysis of the quality of said transmission channel consists indetermining at least one first information selected from:the number ofbursts detected to have been received incorrectly; an estimate of thebit error rate (BER); an estimate of the impulse response of saidtransmission channel; and an estimate of the signal to noise ratio; andduring operation in said second mode of transmission, said analysis ofthe quality of said transmission channel consists in determining atleast one second information selected from:the number of burstsretransmitted; the number of bursts retransmitted detected to have beenreceived incorrectly) an estimate of the bit error rate (BER); anestimate of the impulse response of said transmission channel; and anestimate of the signal to noise ratio.
 13. The method claimed in claim12 wherein, during operation in said first mode of transmission orrespectively said second mode of transmission, the selection of saidsecond or respectively said first mode of transmission comprises a stepof comparing the first or respectively second information representativeof the quality of said transmission channel with at least one first orrespectively second predetermined threshold.
 14. The method claimed inclaim 13 wherein, during operation in said first mode of transmission,said second mode of transmission is selected if said first informationrepresentative of the quality of said transmission channel is below saidpredetermined first threshold, and, during operation in said second modeof transmission, said first mode of transmission is selected if saidsecond information representative of the quality of said transmissionchannel is above said predetermined second threshold.
 15. The methodclaimed in claim 1 wherein an indication of a change from said firstmode of transmission to the second is transmitted over an auxiliarychannel reserved for protocol data.
 16. The method claimed in claim 13wherein an indication of a change from said second mode of transmissionto the first is placed in one of said available bursts.